As it’s fairly common, recommended by Gould, and—depending on house style for longer measures—sometimes necessary to have single measure MMRs marked with a “1” in orchestral parts, it should be documented how this can be achieved:

\set restNumberThreshold = 0

The default seems to be “unset” but in multi-measure-rest-engraver.cc it is hardcoded to 1; you won’t find this value in engraver-init.ly.